With regard to a catalyst for a polymerization of a conjugated diene such as 1,3-butadiene and isoprene, many proposals have been made in the past, and some of them have been industrialized practically. For example, a combination of a compound of titanium, cobalt, nickel, neodymium, or the like, and an organic aluminum compound is often used in a process for producing a conjugated diene polymer having a high proportion of cis-1,4-structure.
Meanwhile, a polymerization of a conjugated diene using an element of the group 3 of the periodic table as the catalyst is also known, and various polymerization processes have been proposed in the past. For example, Patent Literature 1 discloses a catalyst system comprising a salt of a rare-earth metal, an organic metal compound of an element of the groups I to III of the periodic table, and a fluorine-containing organic boron compound. Patent Literature 2 discloses a polymerization catalyst comprising a compound of a metal of the group IIIB of the periodic table, an ionic compound of a non-coordinating anion and a cation, and an organic metal compound of an element of the groups I to III of the periodic table. Patent Literature 3 discloses a process for producing a conjugated diene polymer, wherein a conjugated diene compound is polymerized using a catalyst obtained from a compound of a metal with an atomic number of 57 to 71 having a bulky ligand, an ionic compound consisting of a non-coordinating anion and a cation, and an organic metal compound of an element selected from the groups 2, 12 and 13 of the periodic table. As for the catalysts described in Patent Literatures 1 to 3, however, catalysts which have proved to have the effect in the Examples are mainly neodymium-based catalysts. In addition, Patent Literature 4 discloses a carrier-supported solid catalyst for a (co)polymerization of a conjugated diene, in which at least one compound of a rare-earth metal (specifically, neodymium) from among metals with an atomic number of 57 to 71 or 92 is supported on a carrier.
Non Patent Literature 1 describes the results of polymerizations of diene monomers such as BD (butadiene) using Ln(acac)3.nH2O (Ln: La—Lu) as the catalyst and Et3Al/Et2AlCl as the promoter, which revealed that the polymer was obtained nearly quantitatively with Pr in the polymerization of BD. Non Patent Literature 2 describes the results of polymerizations of butadiene and the results of polymerizations of isoprene using the catalyst system of LnCl3 (Ln: Nd, Pr, Gd).2THF—AlEt3, which revealed that the reactivity was Nd>Pr>Gd.
Meanwhile, a rubber composition comprising a polybutadiene rubber (BR) or a styrene-butadiene rubber (SBR) as the main component, and natural rubber or the like in addition thereto has conventionally been industrially produced and used, in the main, as a material for a tire (rubber for an automobile tire), a crawler of a crawler type traveling device, an industrial rubber belt, and the like, while taking advantage of its characteristics (Patent Literatures 5 to 7). In these applications, characteristics such as low heat build-up and low fuel consumption may be required.
As for a material for a tire, the requirement for the lower fuel consumption of an automobile and the requirement for the safety of driving on snow and ice have increased in recent years, and therefore there is need for the development of a rubber material having a low rolling resistance (i.e., a low tan δ) and a great road surface grip on snow and ice (i.e., wet skid resistance).
However, a rubber having a low rolling resistance such as a polybutadiene rubber (BR) tends to have a low wet skid resistance, while a styrene-butadiene rubber (SBR) has the problem that it has a high wet skid resistance but has a high rolling resistance also. As a method for solving the problems as described above, many technological developments of modified rubbers have been made. Among others, many processes in which a low-cis diene rubber is chemically modified with a modifying agent (for example, a modifying agent containing a functional group to interact with a filler for a rubber composition for a tire, including silica and carbon black) in the presence of a lithium-based catalyst have been proposed.
As an example of a modification of a high-cis diene rubber, Patent Literature 8 discloses a process in which a cis-1,4-polybutadiene is produced using a titanium compound having a cyclopentadienyl skeleton as the catalyst, and then the product is reacted with 4,4′-bis(diethylamino) benzophenone to modify it. However, the ratio (Mw/Mn) of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) is less than 1.5, which is extremely low, and therefore the modified rubber has the problem with processability.
Patent Literatures 9 to 11 disclose that a rubber having an excellent rebound resilience and a low low-temperature hardness can be obtained by reacting a diene rubber having an active alkali metal terminal with a nitroamino compound, a nitro compound, a nitroalkyl compound, or the like. Patent Literatures 12 and 13 disclose processes in which a low-cis BR and SBR are modified with an alkoxysilane compound.
However, a low-cis BR has an insufficient abrasion resistance, and the problem cannot be solved even by the modification. Meanwhile, a SBR has a low rebound resilience, and the drawback cannot be sufficiently resolved even after the modification.
Patent Literatures 14 and 15 disclose Examples in which a high-cis BR obtained by a neodymium catalyst is modified with an alkoxysilane compound. However, the process for producing a modified conjugated diene polymer described in Patent Literature 14 involves the catalyst aging and the synthesis of the intermediate polymer, and therefore the operation is complicated. Meanwhile, the modified polymer described in Patent Literature 15 has a significantly increased Mooney viscosity, and therefore gelation is a matter of concern.
Patent Literatures 16 to 18 disclose processes in which a cis-1,4-polybutadiene is produced using a compound containing a rare-earth element corresponding to the atomic number 57 to 71 of the periodic table as the catalyst, and then the product is reacted with an amine compound, an imide compound, a quinone compound, a thiazole compound, a sulfenamide compound, or the like, to modify it. However, specific examples (Examples) of the process for polymerizing 1,3-butadiene are limited to processes in which a neodymium-containing catalyst is used.
Meanwhile, it is known that polybutadienes having various micro-structures may be obtained depending on the type of the polymerization catalyst, and a polybutadiene which is synthesized using a cobalt compound or a nickel compound and an organic aluminum compound has a high proportion of cis-form (high-cis BR), and therefore is suitably used as a material for a tire. A vinyl-cis polybutadiene (VCR) in which a syndiotactic-1,2-polybutadiene (SPB) is dispersed in a high-cis BR is known as a BR having more highly functional (higher performance) characteristics, while taking advantage of the characteristics of the high-cis BR. The VCR is known as a material which facilitates higher hardness, higher elastic modulus, and improved processability of the product, as compared with a conventional cis-1,4-polybutadiene rubber.
As a process for producing the VCR as described above, Patent Literatures 19 and 20, for example, disclose processes for producing a SPB-containing high-cis BR composite (VCR) using a cobalt catalyst. Patent Literature 21 discloses a process for producing a SPB-containing high-cis BR composite (VCR) using a nickel catalyst. Patent Literatures 22 and 23 also disclose processes for producing a VCR in a similar manner.
In addition, Patent Literature 24 discloses a process for producing a VCR in an inert organic solvent comprising a C4 fraction such as n-butane, cis-2-butene, trans-2-butene, and 1-butene as the main component, wherein a soluble cobalt compound is used as the catalyst for the cis-1,4-polymerization.